Direct C-H cross-coupling approach to heteroaryl coumarins

Article abstract of DOI:10.1039/c2ob07137a

A Pd-catalyzed direct cross-coupling of 3-bromocoumarins with heteroarenes provided an efficient route to synthesizing 3-heteroarylcoumarins. The reaction scope for the transformation was fairly broad, affording modest to good yields of various 3-heteroarylcoumarin scaffolds, which are privileged structures and prevalent motifs in many biologically active compounds and fluorophores. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2ob07137a

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PAPER
Direct C–H cross-coupling approach to heteroaryl coumarins†
Minsik Min, Bomi Kim and Sungwoo Hong*
Received 20th December 2011, Accepted 23rd January 2012
DOI: 10.1039/c2ob07137a
A Pd-catalyzed direct cross-coupling of 3-bromocoumarins with heteroarenes provided an efficient route
to synthesizing 3-heteroarylcoumarins. The reaction scope for the transformation was fairly broad,
affording modest to good yields of various 3-heteroarylcoumarin scaffolds, which are privileged
structures and prevalent motifs in many biologically active compounds and fluorophores.
C–H cross-coupling of a coumarin core with benzothiazoles,
Introduction
benzoxazoles, thiazoles or caffeine would be highly desirable for
a practical route to focused chemical libraries of coumarin
derivatives (Scheme 1). In carrying out these efforts, we estab-
lished an efficient protocol and investigated the photophysical
properties of the synthesized coumarin derivatives, and herein
report the details of these studies.
Coumarins constitute a major class of naturally occurring com-
pounds and privileged medicinal scaffolds that exhibit a broad
range of biological and pharmaceutical properties, including
anti-HIV, anti-tumor, anti-hypertension, anti-arrhythmia, anti-
inflammatory, anti-osteoporosis, antiseptic, analgesic and anti-
coagulant activities.1 Coumarins are also most a important class
of fluorophores, and they have been extensively investigated as
powerful tools for biological applications, especially for live cell
imaging.2 Significant efforts have been directed toward develop-
ing efficient synthetic approaches to coumarin derivatives.3
Among the compounds with valuable optical properties and bio-
logical activities are the 3-heteroarylcoumarins containing ben-
zothiazole, benzoxazole and thiazole residues.4 In this regard,
developing more efficient methods for the construction of this
structural motif is a topic of immense importance. Although
many strategies for the synthesis of 3-heteroarylcoumarins have
been developed,4 the scope of such condensation reactions has
been somewhat limited due to their harsh conditions, multistep
synthesis, poor substituent tolerance, or low chemical yield,
and therefore, the synthesis of some derivatives continues to
pose a challenge. In this regard, developing alternative efficient
methods for the construction of this structural motif is a topic of
immense importance.
Results and discussion
At the outset, the direct C–H cross-coupling reaction conditions
were optimized by employing 3-bromocoumarin (1) and
benzothiazole (2) as model substrates. CuI in the absence of pal-
ladium catalysts could carry out the coupling reaction but exhib-
ited only negligible activity (Table 1, entries 1–2). A catalytic
system comprising both Pd(OAc)2 and CuI improved the reac-
tion efficiency (entry 4). The addition of PPh3 has been shown
to enhance the yield significantly for the coupling reaction (entry
5), indicating the pivotal role of a phosphine ligand in the cataly-
tic cycle. Reaction optimization revealed that, at 110 °C, Pd
(PPh3)4 (2.5 mol%) with CuI (3 equiv), Cs2CO3 (2 equiv) and
1,4-dioxane, was the most efficient in providing the desired
product. These reaction conditions provided an 84% yield of 3a
(entry 6). An excess of the benzothiazole (2 equiv) was needed
to ensure complete conversion of the 3-bromocoumarin. Reac-
tions conducted under other bases, temperatures, solvents or
lower amount of CuI provided lower yields of the cross-coupled
product. Interestingly, replacing CuI with other copper(I)
compounds significantly reduced the reactivity (entry 7).
The direct functionalization of C–H bonds in heteroarenes is
an exceedingly valuable process in the context of contemporary
organic synthesis. In particular, direct arylation and vinylation of
heteroarenes using transition-metal catalysts has found wide-
spread use in synthesis for the construction of complex frame-
works.5 Inspired by the recent advances in this area, we
envisaged that the development of a generally applicable syn-
thetic strategy for preparing 3-heteroarylcoumarins via direct
Department of Chemistry, Korea Advanced Institute of Science and
Technology (KAIST), Daejeon, 305-701, Korea. E-mail: hongorg@kaist.
ac.kr; Fax: (+82) 42-350-2810; Tel: (+82) 42-350-2811
†Electronic supplementary information (ESI) available: 1H and 13C
NMR spectra. See DOI: 10.1039/c2ob07137a
Scheme 1 Synthetic strategy for construction of 3-heteroarylcoumarins
via direct C–H cross-coupling.
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Table 1 Development of cross-coupling of coumarin with
To diversify the 3-heteroarylcoumarin derivatives further, our
method was explored using other heteroarenes. We were pleased
to observe that this synthetic strategy was suitable for the syn-
thesis of 3-thiazolylcoumarins. For example, thiazole and 4,5-
dimethylthiazole participated in a cross-coupling reaction with
3-bromocoumarins to afford the corresponding coupling pro-
ducts in modest to good yields as summarized in Table 4. The
3-thiazolylcoumarin derivatives 6a–6f were found to possess
similar fluorescence properties as those of the corresponding
3-benzothiazolylcoumarins.
Because some xanthine derivatives are biologically active
and fluorescent,7 we further probed our synthetic method for its
utility toward coupling of 3-bromocoumarins with caffeine.
Indeed, the addition of the caffeine group to the 3 position of the
coumarin core was expected to induce a red-shift in the emission
wavelength, by extending the electronic conjugation and introdu-
cing electron-withdrawing effects. To our delight, caffeine was
compatible with the coupling reaction conditions, and modest to
good yields of the desired products were obtained as shown in
Table 5. Furthermore, these new derivatives 7a–7c exhibited
strong photonic luminescence (Table 5) and appeared to be
promising in their potential for further modification to yield bio-
logically active compounds with high photoluminescence
efficiencies.
benzothiazolea
Catalyst
Entry (2.5 mol%)
Yieldb
(%)
Additive (equiv)
CuI (3)
CuI (3); Phen (0.2) 1,4-Dioxane
None
CuI (3)
CuI (3); PPh3 (3)
CuI (3)
Solvent
1
2
3
4
5
6
7
8
9
10
None
None
1,4-Dioxane Trace
7
5
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
1,4-Dioxane
1,4-Dioxane 27
1,4-Dioxane 58
1,4-Dioxane 84
CuCl (3)
1,4-Dioxane
6
Cu(Xantphos)I (3) 1,4-Dioxane 72
CuI (3)
CuI (3)
DMF
Toluene
15
62
a Reactions were conducted with coumarin (1 equiv), benzothiazole
(2 equiv), and Cs2CO3 (2 equiv) at 110 °C for 6 h. b Yields are reported
after isolation and purification by flash silica gel chromatography. Phen
= 1,10-phenanthroline.
No beneficial effects were observed when either Xantphos6
(entry 8) or t-BuCO2H, were employed.
Conclusions
The scope of the Pd-catalyzed direct cross-coupling procedure
was investigated using substituted coumarins and benzothiazoles
or benzoxazoles as coupling partners. Gratifyingly, the reaction
scope using this catalyst system was broad, and all coupling reac-
tions went to completion in less than 12 h with modest to
good yields (Table 2). These derivatives yielded broad fluor-
escence emission spectra over 400–500 nm, associated with high
quantum yields, as summarized in Table 2. The photophysical
and spectroscopic properties of coumarins can be readily
modified by introducing substituents into the coumarin core.4c
The addition of electron-donating groups such as OMe and NEt2
at position 7 on the coumarin core triggers a bathochromic shift
and enhances the photophysical properties. Furthermore, a com-
parative analysis of the values in Table 2 revealed that the substi-
tuent in the benzothiazole or benzoxazole affected the
absorption/emission wavelength. For example, a direct compari-
son of 3a (λem = 451 nm, ΦF = 0.15) with 4d (λem = 491 nm, ΦF
= 0.45) or 4e (λem = 486 nm, ΦF = 0.43) clearly revealed that
introduction of OMe or phenyl groups to the C-6 position on the
benzothiazole ring gave rise to notably red-shifted maxima and
increased the value of ΦF.
Because the photophysical properties also depended on the
substituents on the benzothiazole ring, we carried out a study to
investigate the synergistic effects, if any, on the photolumines-
cence efficiency by incorporating substituents on both the cou-
marin core and the benzothiazole ring. To this end, 7-OMe or
NEt2 substituted coumarins were coupled to 6-OMe or phenyl
substituted benzothiazoles, respectively, under the reaction con-
ditions, to provide the desired products 5a–5d with modest
yields (Table 3). The emission maxima of these compounds
were slightly red shifted relative to the emission maxima of
the corresponding unsubstituted benzothiazolyl coumarins 3e
and 3g.
We developed an efficient method for the direct cross-coupling
of a coumarin core and various heteroarenes via a palladium-cat-
alyzed C–H functionalization reaction. The reaction scope for
the transformation was fairly broad, affording good yields of a
wide range of 3-heteroaryl coumarins, which are privileged
structures in many biologically active compounds and fluoro-
phores. In addition, our synthetic strategy led to the discovery of
new caffeine-based derivatives that show intense photonic
luminescence.
Experimental
General methods and materials
Unless stated otherwise, reactions were performed in flame-dried
glassware under a positive pressure of nitrogen using freshly dis-
tilled solvents. Analytical thin layer chromatography (TLC) was
performed on precoated silica gel 60 F254 plates and visualiza-
tion on TLC was achieved by UV light (254 and 354nm).
Flash column chromatography was undertaken on silica gel
(400–630 mesh). 1H NMR spectra were recorded at 400 MHz or
300 MHz and chemical shifts were quoted in parts per million
(ppm) referenced to the appropriate solvent peak or 0.0 ppm for
tetramethylsilane. The following abbreviations were used to
describe peak splitting patterns when appropriate: br = broad, s =
singlet, d = doublet, t = triplet, q = quartet, m = multiplet, dd =
doublet of doublet. Coupling constants, J, were reported in hertz
(Hz). 13C NMR spectra were recorded at 100 MHz and were
fully decoupled by broad band proton decoupling. Chemical
shifts were reported in ppm referenced to the center line of a
triplet at 77.0 ppm of chloroform-d. Mass spectral data were
obtained from the KAIST Basic Science Institute by using EI
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Table 2 Pd-catalyzed direct cross-coupling of various coumarins with benzoxazole or benzothiazolea
e
Compd
X
S
R1
R2
Yieldb (%)
λabsc (nm)
λemd (nm)
ϕF
3a
3b
3c
3d
3e
3f
H
H
H
H
H
OMe
OMe
NEt2
84
56
47
61
64
40
48
361
393
368
367
379
408
462
451
461
449
458
465
476
496
0.15
0.07
0.28
0.11
0.55
0.73
0.93
OMe
Me
Cl
H
OMe
H
3g
3h
3i
3j
3k
3l
3m
H
Cl
OMe
H
OMe
H
H
H
H
OMe
OMe
NEt2
76
46
64
55
43
50
353
367
382
374
393
445
448
451
460
446
462
481
0.49
0.32
0.03
0.64
0.60
0.93
O
e
Compd
X
S
R1
R2
Yieldb (%)
λabsc (nm)
λemd (nm)
ϕF
4a
4b
4c
4d
4e
Cl
H
H
H
H
H
Me
Cl
OMe
Ph
42
48
33
48
44
364
370
366
371
376
443
465
454
491
486
0.10
0.40
0.49
0.45
0.43
4f
4g
4h
Cl
Me
Ph
H
H
H
81
59
72
378
362
363
441
450
461
0.52
0.61
0.64
O
a Reactions were conducted with coumarin (1 equiv), benzothiazole (2 equiv), Pd(PPh3)4 (2.5 mol%), CuI (3 equiv) and Cs2CO3 (2 equiv) in 1,4-
dioxane at 110 °C over 6–12 h. b Yields are reported after isolation and purification by flash silica gel chromatography. c Only the longest absorption
maxima are shown. d Excited at the maximum excitation wavelength. e Absolute fluorescence quantum yield.
Table 3 Pd-catalyzed direct cross-coupling of substituted coumarins
methods. Commercial grade reagents and solvents were
used without further purification except as indicated below.
Dichloromethane was distilled from calcium hydride. THF was
distilled from sodium. Unless otherwise stated, all commercial
reagents and solvents were used without additional purification.
and benzothiazolesa
General procedure (GP) for synthesis of 3-heteroarylcoumarins
Compd
R1
R2
Yield (%)
λabs (nm)
λem (nm)
ϕF
Bromocoumarins (1 eq.), heteroarene (2 eq.), Pd(PPh3)4 (2.5 mol
%), CuI (3 eq.) and Cs2CO3 (2 eq.) were combined in 1,4-
dioxane under air. The reaction mixture was heated to 110 °C
with stirring. The reaction was monitored by TLC using 25%
EtOAc and 75% n-hexane as the mobile phase. After disappear-
ance of starting material (coumarins), the reaction mixture was
diluted and filtered through Celite with CH2Cl2. After removal
of solvent, the residue was purified by flash chromatography on
silica gel to give desired product.
5a
5b
OMe
Ph
42
40
407
404
489
485
0.46
0.47
OMe
5c
5d
OMe
Ph
36
51
451
454
502
503
0.76
0.78
NEt2
a Reactions were conducted with coumarin (1 equiv), benzothiazole
(2 equiv), Pd(PPh3)4 (2.5 mol%), CuI (3 equiv) and Cs2CO3 (2 equiv) in
1,4-dioxane at 110 °C for 6 h.
2694 | Org. Biomol. Chem., 2012, 10, 2692–2698
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Table 4 Pd-catalyzed direct cross-coupling of coumarins with
119.9, 121.8, 121.8, 122.6, 125.6, 126.7, 136.5, 141.9, 148.5,
151.6, 156.6, 159.9, 160.2; HRMS (EI+) m/z calcd for
C17H11NNaO3S+ [M + Na]+: 332.0352, found: 332.0399.
thiazolesa
3-(Benzo[d]thiazol-2-yl)-6-methyl-2H-chromen-2-one (3c).
Yield 47% (11.6 mg); mp 230–232 °C; IR (KBr): 1737, 1496,
1
1264, 821 cm−1; H NMR (400 MHz, CDCl3) δ 9.01 (s, 1H),
8.07 (d, J = 8.1 Hz, 1H), 7.96 (d, J = 8.1 Hz, 1H), 7.53–7.47
(m, 2H), 7.43–7.40 (m, 2H), 7.31 (d, J = 8.4 Hz, 1H), 2.44
(s, 3H); 13C NMR δ (100 MHz, CDCl3): 21.6, 116.8, 119.0,
120.2, 121.4, 122.3, 125.2, 128.4, 129.4, 133.2, 135.9, 136.9,
141.3, 150.2, 153.8, 158.9, 159.8; HRMS (EI+) m/z calcd for
C17H11NNaO2S+ [M + Na]+: 316.0403, found: 316.0407.
Compd
R1
R2
H
Yield (%)
λabs (nm)
λem (nm)
ϕF
6a
6b
6c
H
OMe
NEt2
60
78
82
349
375
434
423
444
484
0.60
0.70
0.73
3-(Benzo[d]thiazol-2-yl)-6-chloro-2H-chromen-2-one (3d).
Yield 61% (14.9 mg); mp 250–252 °C; IR (KBr): 1727, 1481,
6d
6e
6f
H
OMe
NEt2
64
67
75
366
390
438
456
464
489
0.63
0.61
0.70
Me
1252, 1194, 812 cm−1 1H NMR (300 MHz, CDCl3) δ 8.98
;
a Reactions were conducted with coumarin (1 equiv), benzothiazole
(2 equiv), Pd(PPh3)4 (2.5 mol%), CuI (3 equiv) and Cs2CO3 (2 equiv) in
1,4-dioxane at 110 °C for 6 h.
(s, 1H), 8.09 (d, J = 8.1 Hz, 1H), 7.98 (d, J = 8.1 Hz, 1H), 7.68
(d, J = 2.4 Hz, 1H), 7.58–7.52 (m, 2H), 7.46–7.37 (m, 2H);
HRMS (EI+) m/z calcd for C16H8ClNNaO2S+ [M + Na]+:
335.9856, found: 335.9853.8
3-(Benzo[d]thiazol-2-yl)-7-methoxy-2H-chromen-2-one (3e).
Yield 64% (15.5 mg); mp 232–235 °C; IR (KBr): 1712, 1594,
Table 5 Pd-catalyzed direct cross-coupling of coumarins with
caffeinea
1281, 838, 750 cm−1 1H NMR (400 MHz, CDCl3) δ 9.03
;
(s, 1H), 8.06 (d, J = 8.2 Hz, 1H), 7.95 (d, J = 8.2 Hz, 1H), 7.60
(d, J = 8.8 Hz, 1H), 7.52–7.48 (m, 1H), 7.41–7.37 (m, 1H), 6.93
(dd, J = 8.8, 2.4 Hz, 1H), 6.89 (d, J = 2.4 Hz, 1H); 13C NMR δ
(100 MHz, CDCl3): 56.1, 100.6, 112.7, 114.1, 115.9, 121.8,
122.2, 125.4, 126.7, 130.9, 135.9, 142.7, 151.2, 156.2, 160.1,
160.9, 164.7; HRMS (EI+) m/z calcd for C17H11NNaO3S+
[M + Na]+: 332.0352, found: 332.0371.
Compd
R1
Yield (%)
λabs (nm)
λem (nm)
ϕF
3-(Benzo[d]thiazol-2-yl)-6,7-dimethoxy-2H-chromen-2-one (3f).
Yield 40% (9.4 mg); mp 260–262 °C; IR (KBr): 1713, 1619,
7a
7b
7c
H
OMe
NEt2
46
43
72
352
360
420
479
472
481
0.37
0.53
0.57
1004, 840, 755 cm−1 1H NMR (400 MHz, CDCl3) δ 9.03
;
(s, 1H), 8.05 (d, J = 8.1 Hz, 1H), 7.96 (d, J = 7.7 Hz, 1H), 7.51
(t, J = 7.1 Hz, 1H), 7.40 (t, J = 7.4 Hz, 1H), 7.05 (s, 1H), 6.93
(s, 1H), 3.99 (s, 3H), 3.95 (s, 3H); HRMS (EI+) m/z calcd for
C18H13NNaO4S+ [M + Na]+: 362.0457, found: 362.0467.4c
a Reactions were conducted with coumarin (1 equiv), caffeine (2 equiv),
Pd(PPh3)4 (2.5 mol%), CuI (3 equiv) and Cs2CO3 (2 equiv) in 1,4-
dioxane at 110 °C for 6 h.
3-(Benzo[d]thiazol-2-yl)-7-(diethylamino)-2H-chromen-2-one (3g).
Yield 48% (8.6 mg); mp 213–215 °C; IR (KBr): 2922, 1709,
1
1191, 754 cm−1; H NMR (300 MHz, CD2Cl2) δ 8.92 (s, 1H),
3-(Benzo[d]thiazol-2-yl)-2H-chromen-2-one (3a). Yield 84%
(13.6 mg); mp 221–223 °C; IR (KBr): 2926, 1716, 1604, 1187,
7.97 (t, J = 7.4 Hz, 1H), 7.54–7.46 (m, 2H), 7.37 (t, J = 8.2 Hz,
1H), 6.72 (dd, J = 9.0, 2.5 Hz, 1H), 6.59 (d, J = 2.4 Hz, 1H),
3.49 (q, J = 7.1 Hz, 4H), 1.27 (t, J = 7.1 Hz, 6H); HRMS (EI+)
m/z calcd for C20H18N2NaO2S+ [M + Na]+: 373.0981, found:
373.0985.8
1
756 cm−1; H NMR (400 MHz, CDCl3) δ 9.08 (s, 1H), 8.08
(d, J = 8.1 Hz, 1H), 7.96 (d, J = 8.1 Hz, 1H), 7.72 (dd, J = 7.7,
1.5 Hz, 1H), 7.64–7.62 (m, 1H), 7.54–7.50 (m, 1H), 7.43–7.39
(m, 2H), 7.37 (td, J = 7.6, 0.8 Hz, 1H); 13C NMR δ (100 MHz,
CDCl3): 116.7, 118.9, 120.2, 121.7, 122.9, 125.2, 125.5, 126.6,
129.4, 133.3, 137.8, 141.6, 152.3, 153.9, 159.8, 159.9; HRMS
(EI+) m/z calcd for C16H9NNaO2S+ [M + Na]+: 302.0246,
found: 302.0269.
3-(Benzo[d]oxazol-2-yl)-2H-chromen-2-one (3h). Yield 76%
(9.8 mg); mp 179–181 °C; IR (KBr): 1733, 686 cm−1; 1H NMR
δ (300 MHz, DMSO-d6): 9.07 (s, 1H), 8.00 (dd, J = 7.7, 1.4 Hz,
1H), 7.87–7.76 (m, 3H), 7.51–7.42 (m, 4H) ; HRMS (EI+) m/z
+
calcd for C16H9NNaO3 [M
286.0486.9
+
Na]+: 286.0475, found:
3-(Benzo[d]thiazol-2-yl)-6-methoxy-2H-chromen-2-one (3b).
Yield 56% (13.4 mg); mp 213–215 °C; IR (KBr): 1712, 1455,
1
839, 754 cm−1; H NMR (400 MHz, CDCl3) δ 9.07 (s, 1H),
3-(Benzo[d]oxazol-2-yl)-6-chloro-2H-chromen-2-one (3i).
Yield 46% (6.9 mg); mp 204–206 °C; IR (KBr): 1746, 1250,
8.08 (d, J = 8.2 Hz, 1H), 7.96 (d, J = 8.2 Hz, 1H), 7.52 (td, J =
8.2, 1.2 Hz, 1H), 7.41 (td, J = 8.2, 1.1 Hz, 1H), 7.35 (d, J = 9.1
Hz, 1H), 7.20 (dd, J = 9.1 2.9 Hz, 1H), 7.11 (d, J = 2.9 Hz, 1H);
13C NMR δ (100 MHz, CDCl3): 55.9, 110.7, 117.8, 119.3,
1
814, 674 cm−1; H NMR δ (400 MHz, CD2Cl2): 8.71 (s, 1H),
7.85–7.83 (m, 1H), 7.70 (d, J = 2.4 Hz, 1H), 7.68–7.63 (m, 1H),
7.62 (dd, J = 8.8, 2.5 Hz, 1H), 7.47–7.39 (m, 2H), 7.38 (d, J =
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8.8 Hz, 1H); 13C NMR δ (100 MHz, CD2Cl2): 111.2, 116.7,
118.6, 119.8, 121.0, 125.4, 126.6, 128.6, 130.5, 133.9, 142.1,
144.2, 151.1, 153.3, 156.2, 158.2; HRMS (EI+) m/z calcd for
C16H8ClNNaO3+ [M + Na]+: 320.0085, found: 320.0096.
135.9, 135.9, 136.9, 141.3, 150.2, 153.8, 158.9, 159.8; HRMS
(EI+) m/z calcd for C17H11NNaO2S+ [M + Na]+: 316.0403,
found: 316.0412.
3-(6-Chlorobenzo[d]thiazol-2-yl)-2H-chromen-2-one
(4c).
3-(Benzo[d]oxazol-2-yl)-6-methoxy-2H-chromen-2-one
Yield 64% (7.4 mg); mp 193–195 °C; IR (KBr): 1728,
1256 cm−1 1H NMR δ (400 MHz, CD2Cl2): 8.74 (s, 1H),
(3j).
Yield 33% (9 mg); mp 229–231 °C; IR (KBr): 1725, 1195, 813,
1
744 cm−1; H NMR (400 MHz, CDCl3) δ 9.09 (s, 1H), 7.99
;
(d, J = 8.8 Hz, 1H), 7.93 (d, J = 2 Hz, 1H), 7.73 (dd, J = 7.8, 1.5
Hz, 1H), 7.66–7.63 (m, 1H), 7.49–7.38 (m. 3H); 13C NMR δ
(100 MHz, CDCl3): 116.9, 118.9, 119.7, 121.3, 123.5, 125.4,
127.6, 129.6, 131.6, 133.6, 137.8, 142.2, 150.4, 153.9, 159.8,
160.6; HRMS (EI+) m/z calcd for C16H8ClNNaO2S+ [M + Na]+:
335.9856, found: 335.9846.
7.82–7.81 (m, 1H), 7.65 (d, J = 7.0 Hz, 1H), 7.44–7.40 (m, 2H),
7.34 (d, J = 9.0 Hz, 1H), 7.25 (dd, J = 9.1 Hz, 3.0, 1H), 7.12
(d, J = 2.9 Hz, 1H), 3.88 (s, 3H); 13C NMR δ (100 MHz,
CD2Cl2): 56.3, 111.0, 111.1, 115.9, 118.1, 119.2, 120.9, 122.3,
125.3, 126.4, 142.2, 145.5, 149.6, 151.1, 156.9, 156.9, 158.9;
+
HRMS (EI+) m/z calcd for C17H11NNaO4 [M + Na]+:
316.0580, found: 316.0593.
3-(6-Methoxybenzo[d]thiazol-2-yl)-2H-chromen-2-one
(4d).
Yield 48% (12.8 mg); mp 199–201 °C; IR (KBr): 1726, 1603,
1
821, 758 cm−1; H NMR (300 MHz, CDCl3) δ 9.02 (s, 1H),
3-(Benzo[d]oxazol-2-yl)-7-methoxy-2H-chromen-2-one
(3k).
Yield 55% (8.3 mg); mp 188–190 °C; IR (KBr): 1746, 1613,
7.95 (d, J = 8.9 Hz, 1H), 7.77 (dd, J = 7.7, 1.5 Hz, 1H), 7.64
(t, J = 1.6 Hz, 1H), 7.46–7.40 (m, 3H), 7.14 (dd, J = 8.9, 2.5
Hz, 1H), 3.91 (s, 3H); 13C NMR δ (100 MHz, CDCl3): 55.8,
103.4, 116.7, 116.9, 119.0, 120.2, 123.4, 125.2, 129.3, 133.1,
138.2, 140.8, 146.6, 153.7, 157.4, 158.1, 159.9; HRMS (EI+)
m/z calcd for C17H11NNaO3S+ [M + Na]+: 332.0352, found:
332.0344.
1
1283, 1229, 773, 740 cm−1; H NMR δ (400 MHz, CD2Cl2):
8.72 (s, 1H), 7.80–7.78 (m, 1H), 7.64–7.62 (m, 1H), 7.60 (d, J =
8.6 Hz, 1H), 7.43–7.36 (m, 2H), 6.96–6.93 (m, 1H), 6.89 (d, J =
1.9 Hz, 1H), 3.92 (s, 3H); 13C NMR δ (100 MHz, CD2Cl2):
56.5, 100.9, 111.0, 111.8, 112.6, 113.9, 120.6, 125.1, 126.0,
130.8, 142.3, 145.8, 151.0, 157.1, 157.3, 159.3, 165.2; HRMS
(EI+) m/z calcd for C17H11NNaO4+ [M + Na]+: 316.0580, found:
316.0592.
3-(6-Phenylbenzo[d]thiazol-2-yl)-2H-chromen-2-one
(4e).
Yield 44% (13.7 mg); mp 256–258 °C; IR (KBr): 1738, 1260,
3-(Benzo[d]oxazol-2-yl)-6,7-dimethoxy-2H-chromen-2-one (3l).
Yield 43% (6.8 mg); mp 261–263 °C; IR (KBr): 1726, 1248,
1
797 cm−1; H NMR (300 MHz, CD2Cl2) δ 9.12 (s, 1H), 8.22
(d, J = 1.4 Hz, 1H), 8.13 (d, J = 8.5 Hz, 1H), 7.80 (dd, J = 8.5,
1.8 Hz, 2H), 7.73–7.66 (m, 3H), 7.53–7.40 (m, 5H); 13C NMR δ
(100 MHz, CD2Cl2): 117.0, 119.5, 120.2, 120.8, 123.4, 125.6,
126.6, 127.7, 127.9, 129.9, 133.6, 138.0, 139.1, 140.9, 141.9,
152.4, 154.3, 160.2, 160.4; HRMS (EI+) m/z calcd for
C22H13NNaO2S+ [M + Na]+: 378.0559, found: 378.0561.
739 cm−1 1H NMR δ (400 MHz, CD2Cl2): 8.72 (s, 1H),
;
7.81–7.79 (m, 1H), 7.65–7.63 (m, 1H), 7.41–7.38 (m, 2H), 7.04
(s, 1H), 6.92 (s, 1H), 3.97 (s, 3H), 3.92 (s, 3H); 13C NMR δ
(100 MHz, CD2Cl2): 56.7, 56.9, 100.1, 109.2, 111.0, 111.5,
111.9, 120.6, 125.1, 125.9, 142.4, 142.9, 145.7, 147.5, 151.0,
+
151.9, 155.7, 157.4; HRMS (EI+) m/z calcd for C18H13NNaO5
[M + Na]+: 346.0686, found: 346.0690.
3-(5-Chlorobenzo[d]oxazol-2-yl)-2H-chromen-2-one
(4f).
Yield 81% (11.9 mg); mp 239–241 °C; IR (KBr): 1756, 1000,
3-(Benzo[d]oxazol-2-yl)-7-(diethylamino)-2H-chromen-2-one
(3m). Yield 50% (8.4 mg); mp 179–181 °C; IR (KBr): 1740,
1
802, 754 cm−1; H NMR δ (300 MHz, CDCl3): 8.79 (s, 1H),
1
1587, 1230, 770, 739 cm−1; H NMR δ (300 MHz, CD2Cl2):
7.83 (s, 1H), 7.70–7.66 (m, 2H), 7.56 (d, J = 8.3 Hz, 1H),
7.44–7.37 (m, 3H); HRMS (EI+) m/z calcd for C16H8ClNNaO3
+
8.61 (s, 1H), 7.75 (br, 1H), 7.60 (br, 1H), 7.46 (d, J = 8.9 Hz,
1H), 7.36–7.34 (m, 2H), 6.68 (dd, J = 8.9, 2.3 Hz, 1H), 6.54 (s,
1H), 3.47 (q, J = 7.1 Hz, 4H), 1.25 (t, J = 7.1 Hz, 6H); HRMS
[M + Na]+: 320.0085, found: 320.0097.9
3-(5-Methylbenzo[d]oxazol-2-yl)-2H-chromen-2-one
(4g).
+
(EI+) m/z calcd for C20H18N2NaO3 [M + Na]+: 357.1210,
Yield 34% (10.0 mg); mp 161–163 °C; IR (KBr): 1739,
found: 357.1206.10
1
754 cm−1; H NMR δ (400 MHz, CD2Cl2): 8.77 (s, 1H), 7.71
(dd, J = 7.7, 1.4 Hz, 1H), 7.66 (d, J = 7.3 Hz, 1H), 7.60 (s, 1H),
7.52 (d, J = 8.3 Hz, 1H), 7.42–7.38 (m, 1H), 7.24 (d, J = 9.4
Hz, 1H), 2.49 (s, 3H); 13C NMR δ (100 MHz, CD2Cl2): 21.6,
110.5, 115.8, 117.0, 118.9, 120.6, 125.3, 127.6, 129.6, 134.1,
135.4, 142.4, 145.5, 149.3, 155.0, 156.8, 158.8; HRMS (EI+) m/
3-(5-Chlorobenzo[d]thiazol-2-yl)-2H-chromen-2-one
(4a).
Yield 42% (14.6 mg); mp 271–273 °C; IR (KBr): 1726, 965,
1
750 cm−1; H NMR (400 MHz, CDCl3) δ 9.07 (s, 1H), 8.05
(d, J = 1.8 Hz, 1H), 7.87 (d, J = 8.5 Hz, 1H), 7.73 (dd, J = 7.6,
1.1 Hz, 1H), 7.65 (td, J = 8.5, 1.3 Hz, 1H), 7.43 (d, J = 8.3 Hz,
1H), 7.40–7.37 (m, 2H); HRMS (EI+) m/z calcd for
C16H8ClNNaO2S+ [M + Na]+: 335.9856, found: 335.9858.8
+
z calcd for C17H11NNaO3 [M + Na]+: 300.0631, found:
300.0650.
3-(6-Methylbenzo[d]thiazol-2-yl)-2H-chromen-2-one
(4b).
3-(5-Phenylbenzo[d]oxazol-2-yl)-2H-chromen-2-one
(4h).
Yield 48% (7.0 mg); mp 219–221 °C; IR (KBr): 1722, 1262,
Yield 72% (12.2 mg); mp 174–176 °C; IR (KBr): 1748,
757 cm−1; 1H NMR δ (400 MHz, CDCl3): 8.82 (s, 1H), 8.02 (d,
J = 1.4 Hz, 1H), 7.73 (dd, J = 1.5 Hz, 1H), 7.72–7.65 (m, 5H),
7.50–7.47 (m, 2H), 7.44–7.37 (m, 3H); 13C NMR δ (100 MHz,
CDCl3): 111.2, 115.5, 117.0, 118.8, 119.1, 125.4, 125.9, 127.8,
129.3, 129.7, 134.3, 139.1, 141.1, 142.8, 145.8, 149.5, 150.6,
1
806 cm−1; H NMR (300 MHz, CDCl3) δ 9.03 (s, 1H), 7.95
(d, J = 8.4 Hz, 1H), 7.74 (s, 1H), 7.70 (dd, J = 7.8, 1.4 Hz, 1H),
7.59 (td, J = 7.1, 1.5 Hz, 1H), 7.42 (d, 8.3 Hz, 1H), 7.38–7.31
(m, 2H), 2.50 (s, 3H); 13C NMR δ (100 MHz, CDCl3): 21.6,
116.8, 119.0, 120.2, 121.4, 122.3, 125.2, 128.4, 129.4, 133.2,
2696 | Org. Biomol. Chem., 2012, 10, 2692–2698
This journal is © The Royal Society of Chemistry 2012

View Online
155.0, 156.7, 159.4; HRMS (EI+) m/z calcd for C22H13NNaO3
723 cm−1; H NMR (300 MHz, CDCl3) δ 8.83 (s, 1H), 7.92
(d, J = 3.2 Hz, 1H), 7.55 (d, J = 8.5 Hz, 1H), 7.47 (d, J = 3.2
Hz, 1H), 6.92–6.87 (m, 2H), 3.89 (s, 3H). 13C NMR δ
(100 MHz, CDCl3): 55.9, 100.5, 112.7, 113.8, 116.8, 121.5,
130.2, 139.6, 142.5, 155.5, 159.8, 160.1, 163.8; HRMS (EI+)
m/z calcd for C13H9NNaO3S+ [M + Na]+: 282.0195, found:
282.0203.
+
1
[M + Na]+: 362.0788, found: 362.0799.
7-Methoxy-3-(6-methoxybenzo[d]thiazol-2-yl)-2H-chromen-2-
one (5a). Yield 42% (11.0 mg); mp 205–207 °C; IR (KBr):
1
1719, 827, 816, 767 cm−1; H NMR (300 MHz, CDCl3) δ 8.99
(s, 1H), 7.95 (d, J = 8.8 Hz, 1H), 7.60 (d, J = 8.6 Hz, 1H), 7.37
(d, J = 2.3 Hz, 1H), 7.11 (dd, J = 8.9, 2.5 Hz, 1H), 6.94–6.88
(m, 2H), 3.90 (s, 3H), 3.89 (s, 3H); 13C NMR δ (100 MHz,
CDCl3): 55.9, 56.0, 100.6, 103.5, 112.8, 113.9, 116.4, 116.7,
122.9, 130.6, 137.7, 141.4, 155.9, 157.9, 158.2, 160.2, 164.3;
HRMS (EI+) m/z calcd for C18H13NNaO4S+ [M + Na]+:
362.0457, found: 362.0461.
7-(Diethylamino)-3-(thiazol-2-yl)-2H-chromen-2-one (6c).
Yield 82% (12.1 mg); mp 125–128 °C; IR (KBr): 1706, 1616,
1
819 cm−1; H NMR δ (400 MHz, CD2Cl2): 8.71 (s, 1H), 7.86
(d, J = 3.2 Hz, 1H), 7.46 (d, J = 8.9 Hz, 1H), 7.40 (d, J = 3.2
Hz, 1H), 6.68 (dd, J = 8.9, 2.5 Hz, 1H), 6.56 (d, J = 2.5 Hz,
1H), 3.46 (q, J = 7.1 Hz, 4H), 1.23 (t, J = 7.1 Hz, 6H); 13C
NMR δ (100 MHz, CD2Cl2): 12.6, 45.4, 97.2, 108.8, 110.2,
113.3, 120.3, 130.7, 140.2, 142.7, 152.1, 157.0, 161.2, 161.2;
HRMS (EI+) m/z calcd for C1H16N2NaO2S+ [M + Na]+:
323.0825, found: 323.0842.
7-Methoxy-3-(6-phenylbenzo[d]thiazol-2-yl)-2H-chromen-2-
one (5b). Yield 40% (12.0 mg); mp 268–270 °C; IR (KBr):
1
1714, 1019, 767, 760 cm−1; H NMR (300 MHz, CD2Cl2) δ
9.06 (s, 1H), 8.20 (s, 1H), 8.09 (d, J = 8.4 Hz, 1H), 7.78 (d, J =
9.4 Hz, 1H), 7.71–7.67 (m, 3H), 7.49 (t, J = 7.3 Hz, 2H), 7.39
(t, J = 7.3 Hz, 1H), 6.99–6.95 (m, 2H); 13C NMR δ (100 MHz,
CD2Cl2): 56.1, 77.3, 77.6, 100.5, 112.8, 113.8, 119.8, 122.7,
125.9, 127.3, 127.5, 128.9, 130.7, 137.4, 138.2, 140.5, 141.7,
151.9, 156.1, 164.4; HRMS (EI+) m/z calcd for
C23H15NNaO3S+ [M + Na]+: 408.0665, found: 408.0658.
3-(4,5-Dimethylthiazol-2-yl)-2H-chromen-2-one
(6d). Yield
64% (14.1 mg); mp 190–192 °C; IR (KBr): 1713, 1220, 1191,
1
944, 758 cm−1; H NMR (400 MHz, CD2Cl2) δ 8.75 (s, 1H),
7.68 (dd, J = 7.6 , 1.6 Hz, 1H), 7.61–7.56 (m, 1H), 7.40–7.33
(m, 2H), 2.42 (s, 3H), 2.38 (s, 3H); 13C NMR δ (100 MHz,
CD2Cl2): 11.4, 14.9, 116.8, 119.7, 120.9, 125.3, 129.2, 130.9,
132.4, 137.6, 149.6, 153.6, 154.0, 160.1; HRMS (EI+) m/z calcd
for C14H11NNaO2S+ [M + Na]+: 280.0403, found: 280.0411.
7-(Diethylamino)-3-(6-methoxybenzo[d]thiazol-2-yl)-2H-
chromen-2-one (5c). Yield 36% (9.1 mg); mp 200–203 °C; IR
1
(KBr): 1705, 1256, 1014, 801, 790 cm−1; H NMR (400 MHz,
CD2Cl2) δ 8.83 (s, 1H), 7.86 (d, J = 8.9 Hz, 1H), 7.50 (d, J =
8.9 Hz, 1H), 7.41 (d, J = 2.5 Hz, 1H), 7.08 (dd, J = 8.9 Hz, 2.5
Hz, 1H), 6.70 (dd, J = 8.9, 2.5 Hz, 1H), 6.57 (d, J = 2.4 Hz,
1H), 3.89 (s, 3H), 3.47 (q, J = 7.1 Hz, 4H), 1.25 (t, J = 7.1 Hz,
6H); 13C NMR δ (100 MHz, CD2Cl2): 12.6, 45.4, 56.2, 97.2,
104.0, 108.9, 110.3, 112.9, 116.1, 123.1, 131.0, 138.1, 141.6,
147.7, 152.5, 157.3, 157.8, 159.6, 161.4; HRMS (EI+) m/z calcd
for C21H20N2NaO3S+ [M + Na]+: 403.1087, found: 403.1092.
3-(4,5-Dimethylthiazol-2-yl)-7-methoxy-2H-chromen-2-one (6e).
Yield 67% (15.0 mg); mp 166–168 °C; IR (KBr): 1717, 1016,
1
813 cm−1; H NMR (400 MHz, CD2Cl2) δ 8.70 (s, 1H), 7.58
(d, J = 8.5 Hz, 1H), 6.94–6.89 (m, 2H), 3.09 (s, 3H), 2.41
(s, 3H), 2.37 (s, 3H); 13C NMR δ (100 MHz, CD2Cl2): 11.4,
14.9, 56.3, 100.8, 113.4, 113.7, 117.7, 129.9, 130.3, 138.1,
149.2, 154.7, 155.6, 160.5, 163.8; HRMS (EI+) m/z calcd for
C15H13NNaO3S+ [M + Na]+: 310.0508, found: 310.0510.
7-(Diethylamino)-3-(6-phenylbenzo[d]thiazol-2-yl)-2H-chromen-2-
one (5d). Yield 51% (10.7 mg); mp 212–215 °C; IR (KBr):
7-(Diethylamino)-3-(4,5-dimethylthiazol-2-yl)-2H-chromen-2-
one (6f). Yield 75% (12.2 mg); mp 196–198 °C; IR (KBr):
1
1728, 1275, 760 cm−1; H NMR δ (400 MHz, CD2Cl2): 8.92
1705, 816, 768 cm−1 1H NMR δ (400 MHz, CD2Cl2): 8.58
;
(s, 1H), 8.16 (d, J = 1.8 Hz, 1H), 8.02 (d, J = 8.5 Hz, 1H), 7.73
(dd, J = 8.5, 1.9 Hz, 1H), 7.70–7.68 (m, 2H), 7.52 (d, J = 8.9
Hz, 1H), 7.48 (t, J = 7.4 Hz, 2H), 7.38–7.36 (m, 1H), 6.71 (dd,
J = 8.9, 2.5 Hz, 1H), 6.58 (d, J = 2.4 Hz, 1H), 3.47 (q, J = 7.1
Hz, 4H), 1.25 (t, J = 7.1 Hz, 6H). 13C NMR δ (100 MHz,
CD2Cl2): 12.6, 45.5, 97.2, 109.0, 110.5, 112.6, 120.1, 122.5,
126.1, 127.6, 127.7, 129.3, 131.3, 137.4, 138.0, 141.1, 142.4,
152.5, 152.7, 157.5, 161.3, 162.4; HRMS (EI+) m/z calcd for
C26H22N2NaO2S+ [M + Na]+: 449.1294, found: 449.1284.
(s, 1H), 7.43 (d, J = 8.9 Hz, 1H), 6.66 (dd, J = 9.0, 2.5 Hz, 1H),
6.54 (d, J = 2.5 Hz, 1H), 3.44 (q, J = 7.1 Hz, 4H), 2.36 (d, J =
13.9 Hz, 6H), 1.22 (t, J = 7.1 Hz, 6H); 13C NMR δ (100 MHz,
CD2Cl2): 11.3, 12.6, 14.9, 45.3, 97.2, 109.0, 110.1, 113.7,
128.2, 130.4, 138.7, 148.5, 151.8, 155.9, 156.7, 161.1; HRMS
(EI+) m/z calcd for C18H20N2NaO2S+ [M + Na]+: 351.1138,
found: 351.1147.
1,3,7-Trimethyl-8-(2-oxo-2H-chromen-3-yl)-3,4,5,7-tetrahydro-
1H-purine-2,6-dione (7a). Yield 46% (13.7 mg); mp
3-(Thiazol-2-yl)-2H-chromen-2-one
(6a). Yield
60%
294–296 °C; IR (KBr): 1718, 1656, 760 cm−1 1H NMR
;
(12.0 mg); mp 148–151 °C; IR (KBr): 1730 cm−1
;
1H NMR
(300 MHz, CD2Cl2) δ 8.23 (s, 1H), 7.71–7.68 (m, 2H),
7.45–7.39(m, 2H), 3.95 (s, 3H), 3.56 (s, 3H), 3.37(s, 3H); 13C
NMR δ (100 MHz, CD2Cl2): 28.1, 29.9, 34.2, 109.6, 117.1,
118.1, 125.5, 129.4, 133.9, 146.7, 147.7, 148.4, 151.9, 154.9,
(400 MHz, CD2Cl2) δ 8.91 (s, 1H), 7.97 (d, J = 3.2 Hz, 1H),
7.73 (dd, J = 7.7, 1.4 Hz, 1H), 7.63–7.61 (m, 1H), 7.56 (d, J =
3.2 Hz, 1H), 7.44–7.38 (m, 2H); 13C NMR δ (100 MHz,
CD2Cl2): 116.9, 119.6, 120.8, 122.6, 125.4, 129.6, 132.9, 139.5,
143.5, 153.9, 159.5, 160.2; HRMS (EI+) m/z calcd for
C12H7NNaO2S+ [M + Na]+: 252.0090, found: 252.0098.
+
155.8, 158.7; HRMS (EI+) m/z calcd for C17H14N4NaO4
[M + Na]+: 361.0907, found: 361.0919.
8-(7-(Diethylamino)-2-oxo-2H-chromen-3-yl)-1,3,7-trimethyl-
1H-purine-2,6(3H,7H)-dione (7c). Yield 72% (14.5 mg); mp
7-Methoxy-3-(thiazol-2-yl)-2H-chromen-2-one
(6b). Yield
1
78% (15.4 mg); mp 188–190 °C; IR (KBr): 1710, 1254, 844,
241–243 °C; IR (KBr): 1706, 1663, 1600, 1250, 760 cm−1; H
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 2692–2698 | 2697

View Online
(c) P. T. Thuong, T. M. Hung, T. M. Ngoc, D. T. Ha, B. S. Min,
S. J. Kwack, T. S. Kang, J. S. Choi and K. Bae, Phytother. Res., 2010,
24, 101; (d) R. Dayam, R. Gundla, L. Q. Al-Mawsawi and N. Neamati,
Med. Res. Rev., 2008, 28, 118; (e) G.-T. Kim, K. Lee, H. Kwon and
H.-J. Kim, Org. Lett., 2011, 13, 2799.
3 For selected examples, see: (a) M. S. Schiedel, C. A. Briehn and
P. Bäuerle, Angew. Chem., Int. Ed., 2001, 40, 4677; (b) P. Koenigs,
O. Neumann, K. Hackeloer, O. Kataeva and S. R. Waldvogel,
Eur. J. Org. Chem., 2008, 343; (c) E. Ramesh and R. Raghunathan, Tetra-
hedron Lett., 2008, 49, 1812; (d) B. M. Trost, F. D. Toste and
K. Greenman, J. Am. Chem. Soc., 2003, 125, 4518; (e) J. Oyamada,
C. Jia, Y. Fujiwara and T. Kitamura, Chem. Lett., 2002, 31, 380;
(f) J. Gordo, J. Avó, A. J. Parola, J. C. Lima, A. Pereira and P. S. Branco,
Org. Lett., 2011, 13, 5112.
NMR δ (400 MHz, CD2Cl2): 8.02 (s, 1H), 7.39 (d, J = 8.9 Hz,
1H), 6.67 (dd, J = 8.9, 2.5 Hz, 1H), 6.54 (d, J = 2.4 Hz, 1H),
3.91 (s, 3H), 3.55 (s, 3H), 3.47 (q, J = 7.1 Hz, 4H), 3.36 (s, 3H),
1.24 (t, J = 7.1 Hz, 6H); 13C NMR δ (100 MHz, CD2Cl2): 12.6,
28.0, 29.8, 34.0, 45.4, 97.2, 108.4, 109.0, 110.0, 130.5, 147.7,
148.4, 148.6, 152.0, 152.6, 155.7, 158.0, 159.7; HRMS (EI+)
m/z calcd for C21H23N5NaO4 [M + Na]+: 432.1642, found:
+
432.1660.
Absolute quantum yield measurement
4 (a) S. Lee, K. Sivakumar, W.-S. Shin, F. Xie and Q. Wang, Bioorg. Med.
Chem. Lett., 2006, 16, 4596; (b) D. D. Soto-Ortega, B. P. Murphy,
F. J. Gonzalez-Velasquez, K. A. Wilson, F. Xie, Q. Wang and
M. A. Moss, Bioorg. Med. Chem., 2011, 19, 2596; (c) G. Signore,
R. Nifosi, L. Albertazzi, B. Storti and R. Bizzarri, J. Am. Chem. Soc.,
2010, 132, 1276; (d) A. P. de Silva, H. Q. N. Gunaratne,
T. Gunnlaugsson, A. J. M. Huxley, C. P. McCoy, J. T. Rademacher and
T. E. Rice, Chem. Rev., 1997, 97, 1515.
5 For selected recent examples, see: (a) S. Ueda and H. Nagasawa,
Angew. Chem., Int. Ed., 2008, 47, 6411; (b) S. H. Cho, S. J. Hwang and
S. Chang, J. Am. Chem. Soc., 2008, 130, 9254; (c) W. Liu, H. Cao and
A. Lei, Angew. Chem., Int. Ed., 2010, 49, 2004; (d) H. Ge,
M. J. Niphakis and G. I. Georg, J. Am. Chem. Soc., 2008, 130,
3708; (e) H.-Q. Do and O. Daugulis, J. Am. Chem. Soc., 2007, 129,
12404; (f) R. J. Phipps, N. P. Grimster and M. J. Gaunt, J. Am.
Chem. Soc., 2008, 130, 8172; (g) L.-C. Campeau, D. R. Stuart,
J.-P. Leclerc, M. Bertrand-Laperle, e. Villemure, H.-Y. Sun, S. Lasserre,
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Absolute quantum yields (ФF) were measured by a combination
system of a spectrophotometer (V-530 UV/Vis Spectropho-
tometer, JASCO, Inc.) with a fluorimeter (RF-5301PC Spectro-
fluorophotometer, SHIMADZU Corp.) Samples were prepared
as solutions by dilution of the fluorescent compound in DCM.
Absorption λmax was measured with a spectrophotometer scan-
ning the 700–250 range wavelength while fluorescence emission
λmax and integrated intensity were analyzed with OriginPro 8
software. The absolute quantum yield of a known fluorescent
dye – fluorescein – was obtained to determine those of the
samples precisely (ФF = 0.925 0.015 in 0.1 N NaOH aqueous
solution).
Acknowledgements
This research was supported by National Research Foundation of
Korea (NRF) funded by the Ministry of Education, Science and
Technology (NRF-2010-0022179, 2011-0020322 and 2011-
0016436).
6 J. Huang, J. Chan, Y. Chen, C. J. Borths, K. D. Baucom, R. D. Larsen
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